Method of co-extrusion with foam core

ABSTRACT

This invention relates to a method for a manufacture of plastic sandwich element in which a first extruder is charged with essential ingredients for productions of foamed cellular plastic at a predetermined temperature. A second extruder is charged with essential ingredients for production of solid plastic at a temperature of at least 25 degrees Farenheit below the predetermined temperature. The resulting sandwich is extruded through an extrusion die forming an integral plastic sandwich with the heat of the foamed plastic layer causing a partial melting.

REFERENCE RELATED APPLICATION

This application is continuation-in-part of the application of JosephDukess, Ser. No. 670,623, filed Mar. 26, 1976 METHOD OF CO-EXTRUSIONWITH FOAM CORE now U.S. Pat. No. 4,107,247.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to a method of manufacturing integrally formedplastic sandwiches. More particularly, the invention relates to aprocess for the extrusion of integral plastic members having solid skinsand a cellular inner portion.

2. DESCRIPTION OF THE PRIOR ART

Attempts have been made in the past to provide for the production ofplastic sandwich structural elements by bonding solid plastic skinmembers to the top and bottom surfaces of a cellular plastic core.However, this method of fabrication is costly since it requires theindividual fabrication of the individual skin and core members, theapplication of a relatively costly adhesive material and the clamping ofthe bonded structural element until the adhesive is set. In addition,the difficulty in obtaining a high strength bond over an extendedsurface using an adhesive results in a product having a less thandesirable level of reliability.

Other methods for the fabrication of plastic sandwich members have inthe past included a molding process in which inert gas is dispersed in apolymer melt, the polymer melt is introduced into a mold and as the moldfills, the gas expands within the material forming a cellular core whilea solid plastic skin forms along the walls of the mold. This method hasthe disadvantages of not permitting close control over the skinthickness, not permitting variations in material between the skin andthe core and requiring a large and consequently expensive mold fortheproduction of large plastic members.

Attempts have been made to co-extrude a multi-layer plastic sandwichsuch as disclosed in the Patent to Chisholm et al U.S. Pat. No.3,557,256. This patent no where discloses the concept of providing afoamed core. In Thomas, U.S. Pat. No. 3,331,900 there is disclosed askin coated foamed plastic article. Shippers, U.S. Pat. No. 3,782,870disclosed an apparatus for partially foaming a portion of extrudedmaterial by separating melt streams and then rejoining the separate meltstreams and partially foaming one of the melt streams.

SUMMARY OF THE INVENTION

The present invention contemplates control of the bonding of a cellularcore to the skin material utilizing the concept of temperaturedifferences in the materials being co-extruded. According to the presentinvention, a first extruder is charged with essential ingredients forproduction of cellular plastic at a predetermined temperature. At leastone second extruder is charged with essential ingredients for productionof solid plastic at a temperature differing from and lower than saidpredetermined temperature by at least 25 degrees Farenheit. Thetemperature of the core can be about the melt temperature of the skinfacilitating utilizing and intimate bonding of the core and skinmaterials. Control means are provided for the simultaneous operation ofthe first and second extruders thus resulting in the extrusion of asandwich member having a core of cellular plastic and one or moreintegrally formed skin members of solid plastic. The characteristics ofthe resulting plastic sandwich element can be controlled to feature, asdesired, high flexural strength, hardness relatively high felxuralrigidity, relatively low weight and relatively low cost. By increasingand decreasing the speed of operation of the second extruders, thethickness of skin members may be increased or decreased.

It is therefore an object of the present invention to provide a methodfor the manufacture of an integrally formed coextruded plastic memberhaving a core of cellular plastic and skin members of solid plastic.

Another object of the present invention is to provide an apparatus forthe manufacture of an integrally formed coextruded plastic member havinga core of cellular plastic and skin members of solid plastic with theplastic member having top and bottom skins of a predetermined desiredthickness so as to control the characteristics of the plastic sandwich.

Some of the advantages of the present invention is that the plasticsandwich material so produced is desirable for use as a cap liner. Theproduct prevents product penetration in to the liner when used as aliner for caps, any product penetration stagnating behind the liner andattracting germs. The final product assures a smooth surface in contactwith the contents of the product. The concept of temperaturedifferential co-extrusion of the foam core provides for differentdegrees of resilience while also greatly providing for price advantagein that less material can be used. Further, the layers enable a betterstamped liner allowing a sharp definition while providing for a strongerproduct. The layers encasing the foam core at a various temperature whenco-extruded provide for a desired cell structure while in the event somecells blow too much and are open, the layers will seal these open cellsthus forming a very resilient liner.

These, together with various ancillary objects of the present inventionare obtained by this method of for manufacture of plastic sandwichelements, preferred embodiments being shown in the accompanying drawingby way of example only, wherein:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal sectional view through a portion of anembodiment of an apparatus which can be used according to the presentinvention;

FIG. 2 is a cross sectional view illustrating schematically theconstuction fo a co-extruded plastic sandwich with a retaining thickskin at a temperature of at least 25 degrees Fareheit lower than thecore shown immediately after co-extrusion;

FIG. 3 is a view simlar to FIG. 2 after the core has additionallyfoamed; and,

FIG. 4 and 5 are views similar to FIG. 2 and 3 but illustrating theco-extruded plastic sandwich having a relatively thin skin coextruded ata temperature of at least 25 degrees Fahrenheit lower than the core.

DETAILED DESCRIPTION OF THE INVENTION

With continuing reference to the accompanying drawing, wherein likereference numerals designate similar parts throughout the various views,reference numberal 10 is used to generally designate a typical apparatuswhich can be used for the co-extruding of the plastic sandwich.

As shown three extruders are employed arranged with the extruder 12 forthe core being arranged horizontally and with its auger 14 extendinglongitudinally. The extruders 16 and 20 and their augers 18 and 22 areshown arranged vertically. It is to be understood that while this is aspace saving and covenient typical installation, in accordance with theconcepts of the present invention the extruders can be disposed in anysuitable arrangement such as side by side, angularly disposed, or evenremote from each other the material to be co-extruded being fed to asuitable die 24.

In the illustrated embodiment, the first extruder 12 includes aperforated plate 26 and a tapered nozzle 29 through which core materialC to be extruded is fed to the die 24. Likewise, skin material A feed byauger 18 passes through a perforated plate 30 and through tapered nozzle32 onto the foamed core material C and thence through die 24. The skinmaterial B feed by auger 22 passes through perforated plate 34 andtapered nozzle 36 onto the core material C and thence through die 24.This facilitates the formation of top skin A of solid plastic and abottom skin B of solid plastic on the center core C of cellular plasticof the intergral plastic sandwich member 100. The extruder 12 is chargedwith essential ingredients for the production of cellular plastic andthe second extruder 16 and the third extruder 20 are each charged withessential ingredients for the production of solid plastics.

Control means are provided to control the relative speeds of rotation ofthe augers 14, 18 and 22 to provide for delivery of selected andsufficient quantities of the essential ingredients to provide corematerial C and skin material A and B.

The present invention is adapted to use practically all thermoplasticmaterials. The thickness of the material may range from between 0.010inches to an inch or two while the thickness of the skin material A andfor B is in the order of 0.001 to 0.010 inches, the balance being in thethickness of core material C. A material for the integrally formedplastic sandwich element 100 may be polyethylene which forms A and B ofsolid polyethylene and a core C of closed cell foam polyethylene. Theuse of polyethylene is indicated by way of example, only and otherthermoplastics materials having properties adapted for fabrication by anextrusion process and which can be formed in to a foam or cellular statemay be utilized using the process and apparatus according to the presentinvention. Of course, two, three, or more layers may be used to obtainthe desired thickness and properties where this material is used as acap liner.

EXAMPLE 1

As shown in FIG. 2 and FIG. 3 plastic sandwich 110 is formed with thecore material C being formed from a mixture of melted polyethence,powdered zinc stearate in amounts as conventionally employed as astabilizer and activator. A product sold by Uni-Royal under the tradename of "Celogen A. Z" is employed as a foaming agent. Otherconventional foaming agents may alternatively be employed. The extrusiontemperature of the core material C may range from 300 to 400 degreesFahrenheit at optimum operation and in this example the core materialtemperature selected is 300 degrees Fahrenheit. The skin material A andB employed is delivered at a rate so that the thickness is 0.010 inchesand is relatively thick. While the temperature of this polyethylene mayrange from 200 to 350 degrees Fahrenheit, in this example thetemperature of the skin is 275 degrees Fahrenheit and its melt point isabove 275 degrees but below 300 degrees.

The use of thick skin permits for reasonable tensile strength while theadded heat of the core will promote slight melting of the inner surfacesof the skin in a controlled manner. The thick layer promotes eveness ofthe final sandwich even if the added heat will slightly melt the skinand will allow for sufficient tensile stress and hardness. The slightmelting of the skin will permit foam material at V to slightly penetratethe skin for ensuring a good bond between skin and core.

EXAMPLE 2

In FIGS. 4 and 5 there is shown an embodiment of the invention wherein athin skin of a thickness of 0.001 inch is used at the same temperatureas set forth in Example 1. This embodiment provides for a good expansionfactor in the order of 400% with great resiliency, a shore hardness of65 on a durometer and sufficient tensile strength to function well as acap liner. The added heat of the core thus causes slight melting of theskin and an intimate intermingling of skin and core material at W.

It is to be understood that the skin thickness may be increased ordecreased as desired by increasing or decreasing the output (speed) ofthe extruder for the skin material without making any changes in thetemperature of extruding.

A latitude of modification, substitution and change is intended in theforegoing disclosure, and in some instances, some features of thepresent invention may be employed without a corresponding use of otherfeatures.

What is claimed is:
 1. A method of manufacturing a coextruded article,comprising the steps of: mixing ingredients for a cellular plasticincluding a foaming agent and a thermoplastic material in a plasticstate for forming a core at a predetermined temperature, mixing otheringredients for at least one solid thermoplastic material in a plasticstate at a temperature lower than said predetermined temperature by atleast 25 degrees Fahrenheit for forming a skin for said core, and thensimultaneously coextruding said cellular plastic and said solidthermoplastic material through a die, said predetermined temperaturebeing at least as high as the melt point of said skin to promoteintermingling and intimate bonding of said skin and said core.
 2. Themethod of claim 1 wherein said predetermined temperature is in a rangeof 300 degrees Fahrenheit to 400 degrees Fahrenheit.
 3. A method ofmanufacturing a coextruded article according to claim 1, wherein saidsolid plastic is relatively thin so as to permit enlargement of saidextruded article by foaming after extrusion through said die.
 4. Themethod of claim 3 wherein the thickness of said solid plastic is on theorder of 0.001 inch.
 5. A method of manufacturing a coextruded articleaccording to claim 1, wherein said solid plastic is relatively thick soas to inhibit enlargement of said extruded article.
 6. A method ofmanufacturing a coextruded article according to claim 5, wherein thethickness of said solid plastic is on the order of 0.010 inch so thatthe enlargement of said core is maintained even.